The prior art contains many variations of solid-state power conversion circuits which contain oscillators and switching units based on use of field effect transistors (FETs). For example, U.S. Pat. No. 4,669,038 to Whitford discloses a low power high efficiency power supply using an FET as a chopper. An FET commutated current-fed inverter using thyristors is disclosed in U.S. Pat. No. 4,415,963 to Edwards et al. An FET switching regulator is disclosed in U.S. Pat. No. 4,792,746 to Josephson et al.
A constant current supply is disclosed in U.S. Pat. No. 4,808,909 to Eddlemon, using FETs as a current source, and Zener diodes for bias voltage stabilization. A DC to DC power converter using an energy transfer FET switch is disclosed in U.S. Pat. No. 4,868,730 to Ward. Another DC to DC power converter uses an FET with PWM control to provide voltage regulation, as disclosed in U.S. Pat. No. 5,109,186 to Lieu Fan-Yi.
A power FET having a bipolar transistor acting as an inrush current limiting shunt is disclosed in U.S. Pat. No. 5,010,293 to Ellersick. An FET protection circuit using a Zener diode arrangement for turning on the FET for a load dump is disclosed in U.S. Pat. No. 5,119,265 to Pigott et al. A high voltage power converter using FETs is disclosed in U.S. Pat. No. 4,872,100 to Diaz. A battery power supply circuit for a video light using an FET for power control is described in U.S. Pat. Nos. 5,012,392 and 4,959,755 to Hochstein.
There are known AC power converters for use with indoor illumination loads in the range of 200-400 watts, and these typically use high frequency oscillators having bipolar transistors for high frequency switching. In a 200 watt converter, for example, the bipolar switching transistors used in the oscillator typically switch load current of 4 amperes, and require maintenance of a relatively high base current of 500 mA. For larger loads, such as 400 watts, the transistor needs to carry even greater load current and requires a non-linear increase in base current, to maintain the switching characteristics which drive the transistor into saturation. Larger transistors must be used, and as a result, the transistor driver transformer must be larger and heavier, and this results in increased heat dissipation through increased power consumption and switching losses In both the transistor and transformer.
For indoor illumination loads, the use of larger and heavier components to meet a particular load requirement translates into a larger physical size of the light fixture, and this is undesirable as it complicates mounting arrangements, etc.
Therefore, it would be desirable to provide a high efficiency, high power converter for indoor lighting loads which is relatively small in size and features reduced heat dissipation.